This invention relates to the production of methanol and higher hydrocarbons, i.e. hydrocarbons containing four or more carbon atoms. Higher hydrocarbons may be made from a synthesis gas containing hydrogen and carbon monoxide by the known art of the Fischer-Tropsch process. Likewise methanol is often produced from a synthesis gas containing hydrogen and carbon oxides.
Synthesis gas is normally produced by steam reforming a desulphurised hydrocarbon feedstock, especially natural gas. In this process a mixture of steam and a hydrocarbon feedstock is passed, at an elevated temperature and pressure, through externally heated tubes containing a suitable steam reforming catalyst. Catalysts employed are typically nickel on a suitable support, e.g. alumina, magnesia, zirconia, or calcium aluminate cement. The tubes are heated by a suitable gas, typically the product of combusting a suitable fuel. Typically the temperature is in the range 700 to 950xc2x0 C. and the pressure is in the range 15 to 40, particularly 20 to 30, bar abs. The steam is normally present in an excess over that required for the reforming reaction: in order to reduce the risk of formation of carbon deposits on the reforming catalyst, the steam ratio is typically in the range 1.5 to 4, especially 2 to 3.5. By the term steam ratio is meant the number of gram moles of steam per gram atom of hydrocarbon carbon in the feedstock. The reformed gas will contain hydrogen, carbon monoxide, carbon dioxide, unreacted steam and methane: the precise composition will depend on a variety of factors including the pressure, temperature, and composition of the hydrocarbon/steam mixture. Normally the reformed gas is cooled, with heat recovery, to below the dew point of the steam therein to condense the unreacted steam which is then separated, leaving the residual reformed gas as the synthesis gas.
The Fischer-Tropsch process is often operated by passing the synthesis gas at an elevated temperature and pressure, for example 30 to 50, particularly 35 to 45, bar abs. through a reactor wherein it is contacted with a catalyst, usually an iron- or cobalt-containing composition: a mixture of hydrocarbons is formed together with water and the water and higher hydrocarbons are separated from the residual gas. Conventionally, part of the residual gas is recycled to the reactor as part of the synthesis gas feed, while the remainder of the residual gas is taken as a purge. For the production of hydrocarbons, a stoichiometric synthesis gas has a hydrogen to carbon monoxide molar ratio of about 2, but in order to achieve a high conversion of carbon monoxide, the reactor is often operated with a carbon monoxide-rich gas, i.e. having a hydrogen to carbon monoxide molar ratio below 2, for example having a hydrogen to carbon monoxide molar ratio in the range 1.4 to 1.8, especially 1.4 to 1.6.
Carbon dioxide is largely inert in the Fischer-Tropsch reaction, although some Fischer-Tropsch catalysts exert some activity for the shift and reverse shift reactions:
CO+H2O ⇄CO2+H2
As a result of the presence of carbon dioxide in the synthesis gas fed to the Fischer-Tropsch reaction and possibly the production of carbon dioxide by the shift reaction by the Fischer-Tropsch catalyst, the purge gas from the Fischer-Tropsch stage contains carbon dioxide as well as some hydrogen and carbon monoxide.
Methanol is normally synthesised from a synthesis gas containing hydrogen and carbon oxides by passing the synthesis gas over a suitable catalyst at an elevated temperature and pressure. The synthesis gas is normally produced by steam reforming as described above. Normally a copper-containing catalyst is employed: suitable catalysts include compositions containing copper, zinc oxide, chromia and/or alumina and possibly other oxidic materials such as magnesia. The reaction is typically operated at temperatures above 200xc2x0 C. and at pressures in the range 50 to 150, especially 70 to 120, bar abs. The methanol synthesis is generally effected in a loop wherein the synthesis gas as xe2x80x9cmake-upxe2x80x9d gas is mixed with recycled gas, and the mixture fed to the synthesis reactor. The reacted gas from the synthesis reactor is cooled to condense methanol which is then separated and then the residual gas recycled as the recycle gas. To avoid a build-up of inerts, some of the residual gas is taken as a purge. In the methanol synthesis reaction, methanol is synthesised from both carbon monoxide and carbon dioxide. A gas that is stoichiometric for methanol synthesis has a xe2x80x9cRxe2x80x9d value of 2 where
R=([H2]xe2x88x92[CO2])/([CO]+[CO2])
where [H2], [CO2] and [CO] respectively are the molar proportions of hydrogen, carbon dioxide and carbon monoxide.
I have realised that the residual gas from the Fischer-Tropsch reaction may be used as some or all of the feed to a methanol synthesis process: in this way, use may be made of the carbon dioxide, as well as hydrogen and carbon monoxide, in the residual gas from the Fischer-Tropsch process.
Accordingly the present invention provides a process for the co-production of methanol and higher hydrocarbons by synthesising the hydrocarbons from a synthesis gas containing hydrogen, carbon monoxide and carbon dioxide by the Fischer-Tropsch reaction, separating the higher hydrocarbons, and synthesising methanol from the residual gas.
Thus in the present invention, the residual gas from the Fischer-Tropsch stage is used for the synthesis of methanol. As a result the Fischer-Tropsch reaction may be operated on a xe2x80x9conce-throughxe2x80x9d basis, rather than employing a recycle of part of the residual gas remaining after separation of higher hydrocarbon products from the reacted gas from the Fischer-Tropsch reaction.
The synthesis gas resulting from steam reforming of a hydrocarbon feedstock such as natural gas, as described above, will generally have a hydrogen to carbon monoxide ratio of at least 3, and often in the range 4 to 6. Also it generally has a xe2x80x9cRxe2x80x9d value above 2.5. In order to render the synthesis gas more suited to the Fischer-Tropsch reaction, some of the hydrogen may be separated, for example by a membrane system, in one or more stages, to give a synthesis gas having a hydrogen to carbon monoxide ratio below 2.5, and preferably below 2, e.g. in the range 1.4 to 1.8. When using a membrane separation system, a small amount of the carbon dioxide may also permeate through the membrane with hydrogen. Some or all of the separated hydrogen may be used as fuel, e.g. that combusted to heat the reformer tubes.
The process of the invention may also be employed where steps are taken to produce a reformed gas having a xe2x80x9cRxe2x80x9d value closer to 2. Thus processes are known for the production of methanol synthesis gas wherein the reformed gas is subjected to a stage of partial combustion with oxygen. This has the effect of removing some of the excess of hydrogen by forming water and hence decreasing the value of xe2x80x9cRxe2x80x9d. Since the partial combustion raises the temperature of the gas it is also possible to decrease the residual methane content of the synthesis gas and hence the amount of inerts fed to the synthesis step. In some known processes, the hot partially combusted gas is used as the heating medium for the reformer tubes.
Whether or not hydrogen is separated to adjust the hydrogen to carbon monoxide ratio to render the synthesis gas more amenable to the Fischer-Tropsch reaction, it will normally be necessary to compress the synthesis gas somewhat from the pressure at which the reforming was effected. Thus while the reforming is preferably effected at a pressure in the range 20 to 30 bar abs., the Fischer-Tropsch reaction is preferably effected at a pressure in the range 35 to 45 bar abs.
In a preferred process the synthesis gas is produced by reforming and separation of the excess of steam as described above and then passed through a membrane separation unit to remove some hydrogen, and then at least some of the non-permeate from the membrane separation unit is compressed and at least some of the compressed gas is passed through a second membrane separation unit to separate a further amount of hydrogen. Some of the non-permeate from the first membrane separation unit may bypass the second membrane separation unit: the pressure energy in the non-permeate from the second membrane separation unit may be used to compress the gas which has bypassed the second membrane separation unit.
The non-permeate from the second membrane unit, preferably with any bypass thereof, is used as the feed to the Fischer-Tropsch reactor and the off-gas, i.e. residual gas after separation of the higher hydrocarbons, from the Fischer-Tropsch reaction is used to make methanol, e.g. by use as make-up gas for a methanol synthesis loop, preferably after further compression. Where hydrogen is separated from the reformed gas prior to the Fischer-Tropsch synthesis, it is generally desirable to re-introduce some or all of this hydrogen into the Fischer-Tropsch off-gas prior to methanol synthesis. Where the hydrogen is separated by means of two membrane separation units with compression between the two separation stages as described above, it is preferred that the hydrogen added to the off-gas prior to methanol synthesis, is at least part of the hydrogen separated in the second separation stage.
The Fischer-Tropsch reaction is well known and may be operated using any of the usual technologies and reactor designs. Preferably the Fischer-Tropsch reactor is of the slurry reactor type. The Fischer-Tropsch reaction is exothermic and heat is usually recovered from the reaction, e.g. by the provision of suitable cooling coils. The recovered heat may be used to raise steam. Unlike conventional Fischer-Tropsch reactions, however, as described above, in the present invention it is preferred not to recycle part of the off-gas to the Fischer-Tropsch synthesis. In the Fischer-Tropsch reactor some of the carbon monoxide and hydrogen combine to form hydrocarbon compounds together with by-product water. Preferably the Fischer-Tropsch reaction is effected under such conditions that about 50% to 80%, especially 50% to 65%, of the carbon monoxide in the feed to the Fischer-Tropsch reactor is reacted to hydrocarbons. The products of the Fischer-Tropsch reaction range from methane through liquid hydrocarbons to waxes. The condensed products may mainly comprise C4 to C20 compounds. The hydrocarbons are largely straight chained paraffins. The higher hydrocarbon product is separated from the Fischer-Tropsch catalyst and the residual gas is extracted from the Fischer-Tropsch reactor by means known in the art. The residual gas may be cooled to separate and recover some of the more volatile hydrocarbon products and the water formed during the Fischer-Tropsch reaction. Methane, and other lower hydrocarbons, are generally not separated from the gas stream but can be used as fuel and/or at least part recycled as described hereinafter and/or used as part of The reformer feed. The higher hydrocarbon products may be separated by distillation into a LPG fraction, naphtha, diesel and wax. Often the higher molecular weight components, e.g. waxes, may be subjected to mild hydro-cracking and isomerisation processes. For hydro-cracking, hydrogen is required. This may be supplied from hydrogen separated from the reformed gas prior to the Fischer-Tropsch reaction and/or from hydrogen separated from the purge gas from the methanol synthesis stage. The main cracked product resulting from hydro-cracking of the wax fraction is diesel, which is preferably returned to a suitable place in the Fischer-Tropsch products separation section. Using this process, a sulphur-free uncracked diesel of cetane number of about 80, together with a cracked diesel of cetane number of about 65 can be produced. Preferably the LPG fraction is recycled back to the Fischer-Tropsch reactor. The naphtha fraction has a very straight chain and is particularly suitable for ethylene production.
The off-gas from the Fischer-Tropsch reaction is then used for methanol synthesis. Normally it will be sent to the make-up gas compressor of the methanol synthesis stage and from there into a methanol synthesis loop.
The methanol synthesis stage is normal although the gas composition may be richer in inerts (methane) and carbon dioxide than is normally encountered in methanol synthesis. In spite of the relatively higher carbon dioxide content of the synthesis gas fed to the methanol synthesis loop, efficient methanol synthesis may be achieved since the synthesis gas may have a composition closer to stoichiometric for methanol synthesis than in a normal methanol synthesis loop. The increased carbon dioxide content will also result in the production of a greater amount of water than is normal in methanol synthesis and hence increase the distillation duty. However, heat recovered from the Fischer-Tropsch reaction may be used to supply energy required for such distillation. As a result of the present invention, it is likely that traces of higher hydrocarbons will be present in the off-gas fed to the methanol synthesis stage and hence may appear in the product methanol, and be detectable even after the usual distillative purification stages. To avoid a large inerts content in the methanol synthesis loop, a larger purge than normal may be required. Hydrogen may be recovered from the purge for use in a hydro-cracker as indicated above. Part of the purge may be recycled as part of the hydrocarbon feedstock to the reformer: In this way part of the carbon xe2x80x9clostxe2x80x9d by formation of lower hydrocarbons such as methane in the Fischer-Tropsch process can be recovered.
The process of the invention may be applied to an existing methanol or Fischer-Tropsch plant. Thus if there is no economic market for part of the output of an existing methanol plant, for example as a result of global or local over-capacity, it may be more economic to convert the plant to operate the present invention rather than to run the unmodified plant at a reduced throughput. Where there are two adjacent methanol plants, by adoption of the present invention, the reforming stages of both plants may be used, but the synthesis loop of one plant closed down. Thus in the event that about half of the carbon in the original synthesis gas produced from the reforming stages of the adjacent plants is converted to Fischer-Tropsch products then the off-gas from the Fischer-Tropsch stage can be compressed to the desired methanol synthesis pressure using the synthesis gas compressor of only one of the adjacent plants and fed to the synthesis loop of that plant. Despite the increased carbon dioxide content of the synthesis gas fed to the loop, it should still be possible to maintain the output from the one operating synthesis loop at close to its original output partly because the loop would then be running at closer to its stoichiometric ratio. Where, as described above, prior to feeding to the Fischer-Tropsch synthesis, some or all of the reformed gas, possibly after separation of some of the hydrogen, is compressed, the compression may be effected in the synthesis gas compressor of the other of the adjacent methanol plants. This method of integrating the Fischer-Tropsch process with two methanol plants has the advantage of allowing the Fischer-Tropsch plant to be built and then only requiring a very short shutdown of the methanol plants to tie in the Fischer-Tropsch plant to the methanol plants.
In applying the invention to an existing methanol plant or plants, other changes that may result may be the use of separated hydrogen as fuel for the steam reforming stage, and as the hydrogen-containing gas often required to be added prior to desulphurisation of the feedstock.
If this idea is applied to integrating two adjacent 2500 tonnes per day (tpd) methanol plants, then about 1000 to 1100 tpd of hydrocarbons can be produced in the Fischer-Tropsch stage. Steam from the Fischer-Tropsch heat of reaction may be used to replace any methanol synthesis loop heat lost because of the lower methanol production rate and because the methanol will now be produced more from carbon dioxide than from carbon monoxide, e.g. to provide energy for a natural gas saturator in one of the synthesis gas generation units, or to provide some or all of the heat needed to separate the Fischer-Tropsch products, preferably by distillation. About 1000 to 1100 tpd of water is also produced by the Fischer-Tropsch reaction and may be used to reduce the need to import water for the plant. In round terms this would halve the water make-up requirements. Where the steam required for reforming is introduced into the hydrocarbon feedstock by saturation, i.e. where the feedstock is contacted with a stream of heated water under pressure, the water produced by the Fischer-Tropsch reaction could be used as part of the water used for saturation: This would serve to recycle any dissolved or entrained hydrocarbons.